In the prior art ceramics having hexagonal crystalline structures such as carbides and nitrides, having melting temperatures on the order of 2000 to 3000 degrees C., have been used traditionally as electrical insulators (dielectrics) for circuit boards, chip-carriers and the like. However, they have not heretofore been known to exhibit conductive or semiconductive properties when exposed to thermal heat processing in an air atmosphere in connection with their traditional uses or applications.
More specifically, in the prior art conductive patterns on polymer or ceramic substrates such as, printed circuit boards typically are used to provide interconnections between devices attached to the substrates. Various methods have been used for the production of conductive patterns, such as dry-film imaging and screen printing; for further details on these processing technologies see, e.g., the book by R. H. Clark, Handbook of Printed Circuit Manufacturing, Van Nostrand Reinhold Co., 1985.
More recent prior art methods may use a laser to assist in the production of the conductive patterns on such substrates; several of these techniques are briefly described hereinbelow with reference to known patented techniques:
In U.S. Pat. No. 4,159,414, issued Jun. 26, 1979, to Nam P. Suh, et. al., entitled "Method for Forming Electrically Conductive Paths", there is disclosed a method for forming electrically conductive paths on a substrate made of a composition of a polymer material having preferrably a metal oxide incorporated as a filler therein. A selected surface is heated by a high intensity laser beam, at the locations where the paths are to be formed to a temperature sufficient to reduce the metal compound to its elemental state whereby the desired electrically conductive paths are formed.
In U.S. Pat. No. 4,496,607, issued Jan. 29, 1985, to Eckart Mathias, entitled "Laser Process for Producing Electrically Conductive Surfaces on Insulators", there is disclosed a process for producing electrically conductive surfaces on insulating substrates using a laser beam to melt tracks onto the substrate into which conductive metal particles are simultaneously impinged, thereby resulting in the formation of computer controlled patterns designed according to tracings characteristic of those used in the manufacture of electric/electronic circuit boards.
In U.S. Pat. No. 4,691,091, issued Sep. 1, 1987, to Alan M. Lyons, et. al., entitled "Direct Writing of Conductive Patterns", there is disclosed a method for producing electrically conductive paths on a polymeric substrate by laser writing, i.e. tracing desired paths on the substrate by a laser beam. The resulting paths are conductive carbon as produced by thermal decomposition of substrate surface material. The paths can serve as electrical interconnections akin to printed circuitry on a wiring board. Optionally, the conductivity of the paths may be enhanced by electroplating a suitable conductor metal or alloy onto the paths.
In U.S. Pat. No. 4,710,253, issued Dec. 1, 1987, to Peter Soszek, entitled "Method for Manufacturing a Circuit Board", there is disclosed a circuit board manufactured by applying a film of heat activated adhesive to a substrate and depositing on the film a layer of a conductive powder. The powder and film are then activated by laser radiation. The excess powder is then removed and the substrate fired to cure or bond the powder to the substrate while removing excessive adhesive film.
In U.S. Pat. No. 4,847,138, issued Jul. 11, 1989 to Elizabeth A. Boylan, et. al., entitled "Thermal Writing on Glass and Glass-Ceramic Substrates", there is disclosed a method of producing a transition metal pattern on a glass or glass-ceramic substrate by selective exudation (precipitation) of a transition metal from a glass containing the metal as an oxide. The selective exudation is effected by applying an intense, well focused source of energy to the glass in a pattern corresponding to the desired metal pattern. This develops localized heating, and thereby causes corresponding localized metal exudation from the glass. In this patent the substrate is preferrably loaded with 1-20 percent metal oxide that is known to exude when exposed to localized thermal exposure.
In U.S. Pat. No. 4,880,770, issued Nov. 14, 1989, to Jose M. Mir, et. al., entitled "Metalorganic Deposition Process for Preparing Superconducting Oxide Films", there is disclosed the use of a laser to anneal metal oxide ceramic to recrystallize an amorphous layer to make that layer electrically conductive. This prior art patent relates to the production of super-conducting oxide films which are well known as exhibiting electroconductive properties only at cryogentic temperatures, and therefore, is instructive herein as to the use of lasers to provide localized thermal exposure in preselective pattern areas for annealing purposes.
In U.S. Pat. No. 4,912,087, issued Mar. 27, 1990, to Mohammad Aslam, et. al., entitled "Rapid Thermal Annealing of Superconducting Oxide Precursor Films on Si and SiO.sub.2 Substrates", there is disclosed a method of preparing a superconducting metal oxide film on silicon and silicon dioxide substrates. A superconducting metal oxide precursor is deposited directly on the substrate by vapor depositon and then subjecting it to rapid thermal annealing in an oxygen atomosphere so that a superconducting metal oxide film on silicon and silicon dioxide is formed. The resulting products formed are superconducting metal oxides which are operable only at cryogenic temperatures that is different from the present invention which is operable at non-cryogenic temperatures.
In another example of the prior art, see Laser and Applications, Sep. 1986, page 40, where there is disclosed a process where a chip laden circuit board is placed in a sealed, chemical-vapor-deposition cell. After the cell is pumped out, a gas, for example tungsten-hexafluoride, is fed in. A laser is then focussed through a window in the cell and onto the circuit board. As the laser beam impinges along a prescribed path on the board surfaces, it strips tungsten from the gas and deposits it as a thin metallic line on the surface of the sample.
As can readily be appreciated from the foregoing discussions of the prior art, it is desireable to find a method and materials by which conductive or semiconductive path or surfaces can be readily adapted for the formatiom of conductive or semiconductive surfaces heretofore unknown.